Retinal microsurgery is one of the most challenging set of surgical tasks due to human sensory-motor limitations, the need for sophisticated and miniature instrumentation, and the inherent difficulty of performing micron scale motor tasks in a small and fragile environment. In retinal surgery, surgeons are required to perform micron scale maneuvers while safely applying forces to the retinal tissue that are below sensory perception. Surgical performance is further challenged by imprecise instruments, physiological hand tremor, poor visualization, lack of accessibility to some structures, patient movement, and fatigue from prolonged operations. The surgical instruments in retinal surgery are characterized by long, thin shafts (typically 0.5 mm to 0.7 mm in diameter) that are inserted through the sclera (the visible white wall of the eye). The forces exerted by these tools are often far below human sensory thresholds.
The surgeon therefore must rely on visual cues to avoid exerting excessive forces on the retina. These visual cues are a direct result of the forces applied to the tissue, and a trained surgeon reacts to them by retracting the tool and re-grasping the tissue in search of an alternate approach. This interrupts the peeling process, and requires the surgeon to carefully re-approach the target. Sensing the imperceptible micro-force cues and preemptively reacting using robotic manipulators has the potential to allow for a continuous peel, increasing task completion time and minimizing the risk of complications. All of these factors contribute to surgical errors and complications that may lead to vision loss.
An example procedure is the peeling of the epiretinal membrane, where a thin membrane is carefully delaminated off the surface of the retina using delicate (20-25 Ga) surgical instruments. The forces exerted on retinal tissue are often far below human sensory thresholds. In current practice, surgeons have only visual cues to rely on to avoid exerting excessive farces, which have been observed to lead to retinal damage and hemorrhage with associated risk of vision loss.
Although robotic assistants such as the DAVINCI™ surgical robotic system have been widely deployed for laparoscopic surgery, systems targeted at microsurgery are still at the research stage. Microsurgical systems include teleoperation systems, freehand active tremor-cancellation systems, and cooperatively controlled hand-over-hand systems, such as the Johns Hopkins “Steady Hand” robots. In steady-hand control, the surgeon and robot both hold the surgical tool; the robot senses forces exerted by the surgeon on the tool handle, and moves to comply, filtering out any tremor. For retinal microsurgery, the tools typically pivot at the sclera insertion point, unless the surgeon wants to move the eyeball. This pivot point may either he enforced by a mechanically constrained remote center-of-motion or software. Interactions between the tool shaft and sclera complicate both the control of the robot and measurement of tool-to-retina forces.
To measure the tool-to-retina forces, an extremely sensitive (0.25 mN resolution) force sensor has been used, which is mounted on the tool shaft, distal to the sclera insertion point. The three sensor allows for measurement of the tool tissue forces while diminishing interference from tool-sclera forces.
In addition, a first-generation steady-hand robot has been specifically designed for vitreoretinal surgery. While this steady-hand robot was successfully used in ex-vivo robot assisted vessel cannulation experiments, it was found to be ergonomically limiting. For example, the first generation steady-hand robot had only a ±30% tool rotation limit. To further expand the tool rotation range, a second generation steady-hand robot has been developed which has increased this range to ±60%. The second generation steady-hand robot utilizes a parallel six-bar mechanism that mechanically provides isocentric motion, without introducing large concurrent joint velocities in the Cartesian stages, which occurred with the first generation steady-hand robots.
The second generation steady-hand robot incorporates both a significantly improved manipulator and an integrated microforce sensing tool, which provides for improved vitreoretinal surgery. However, because of the sensitivity of vitreoretinal surgery, there is still a need in the art for improved control of the tool, to avoid unnecessary complications. For example, complications in vitreoretinal surgery may result from excess and/or incorrect application of forces to ocular tissue. Current practice requires the surgeon to keep operative forces low and safe through slow and steady maneuvering. The surgeon must also rely solely on visual feedback that complicates the problem, as it takes time to detect, assess and then react to the faint cues; a task especially difficult for novice surgeons.
Accordingly, there is a need in the art for an improved control method for surgical tools used in vitreoretinal surgery and the like.